1. Field of the Invention
The present invention relates to a received packet processing method and a received packet processing method for processing a packet received from a transmitter via an IP (Internet Protocol) network.
2. Description of the Related Art
Recently, broadcasting is performed by transmitting radio waves in a VHF band or UHF band from a broadcasting station and receiving the radio waves at a receiver. Namely, broadcasting usually and generally means wireless broadcasting.
However, because of recent development of IP network technology, wired broadcasting has been able to be performed using an IP network. According to the IP network technology, an RTP (Real-time Transport Protocol) packet according to RFC3550 is inserted into a payload of an IP packet and image data is inserted into the RTP packet. In many cases, the image data is compressed image data according to MPEG (Moving Picture Experts Group) or the like.
There are related documents as follows: JP-A-2000-307637 and JP-A-2000-228676
If the receiver is to receive broadcasting data on a certain channel via the IP network, the broadcasting data on the channel is not always transmitted from one broadcasting station. For example, a local station broadcasts the broadcasting data received from a Tokyo station in one time zone and broadcast the broadcasting data created by the local station in another time zone.
In one broadcasting station, it is possible to create a time stamp using a date distributed from one common time stamp clock and insert the created time stamp into a header of an RTP packet. Accordingly, if a plurality of programs created in one broadcasting station is combined, it is possible to ensure continuity of time stamps among the programs. It is also possible to ensure continuity of sequence numbers inserted into headers of respective RTP packets among the programs.
However, as stated, if a plurality of broadcasting data transmitted from broadcasting stations different according to time zones is to be combined, it is disadvantageously difficult to ensure continuity of time stamps and continuity of sequence numbers.
Under these circumstances, if the related technique is applied to a receiver, a packet just after changeover from broadcasting data in one time zone to that in a next time zone is abandoned for the following reason. If sequence numbers are discontinuous before and after the changeover, a packet the sequence number of which is not continuous to that of a previously received packet is regarded as an unauthorized packet.
To avoid the disadvantage, if the packet the sequence number of which is not continuous to that of the previously received packet is regarded as an authorized packet and transferred to a higher layer as it is, the packet right after the changeover from the broadcasting program in one time zone to that in the next time zone can be certainly relieved.
In the meantime, according to a method of transmitting IP packets on the IP network, it is not guaranteed that an order of IP packets arriving at a receiver is the same as that for transmitting the IP packets from a transmitter. Instead, an IP packet transmitted later from the transmitter often arrives at the receiver at earlier time than that transmitted earlier from the transmitter. In that case, the continuity of sequence numbers inserted into the headers of the RTP packets is destroyed, with the result that rearrangement of sequence numbers occurs.
If the rearrangement of sequence numbers resulting from characteristics of the IP network occurs to the same broadcasting program, the packet the sequence number of which is not continuous to that of the previously received packet is regarded as an authorized packet, and the packet is transferred to the higher layer as it is, then the receiver is unable to correctly receive the broadcasting program. In this case, it is disadvantageously necessary to rearrange the order of packets so as to recover the continuity of sequence numbers.